DC/DC converters are electronic devices that use switching devices to transform voltage from one level into another level. Typically, the output voltage is regulated and protected against short circuits. The input and output potentials may be galvanically isolated from each other or, they may have a common galvanic connection, and so be non-isolated from each other.
DC-DC converters whose input is non-isolated from its output tend to be more power efficient (i.e., have less power loss) than isolated DC-DC converters.
Among many applications, these types of devices are used in spacecraft, satellites and in high energy physics instrumentation. In these specific applications, the DC/DC converters are subjected to many forms of radiation damage.
FETs (Field Effect Transistors) used for power switching are usually enhancement mode types. This means that they are normally non-conducting. When a gate voltage above a threshold is applied, the FET becomes conducting. FETs are available in two gate polarities; N channel and P channel.
DC/DC converters designed for general purpose use are usually constructed with N channel FETs because, for any given die size transistor, the N channel FET has a lower on resistance than a correspondingly sized P channel FET would have.
Presently available radiation tolerant DC/DC converters use specially designed radiation hardened N channel FETs for switching. The principal benefit of these parts is that the gate threshold voltage doesn't change much after being exposed to radiation. However, these parts have limited sources, are expensive and may have long lead times, leading to higher prices and longer delivery times for the radiation tolerant DC/DC converters that incorporate these types of parts.
Over the past several decades, many standard integrated circuits have been developed to provide drive signals for DC/DC converters and switching power supplies. Existing integrated circuits used to directly drive power transistors in DC/DC converter applications are designed to operate with N channel FETs.
When conventional non-radiation hardened N Channels FETs are used in applications where radiation is present, the application tends to fail at relatively low radiation levels because the gate threshold voltage of the N channel FET shifts more negatively, and ultimately falls close to zero. At this point, the N channel FET conducts current with little or no gate voltage applied. Therefore, the part is uncontrollable.
The gate threshold voltage of a conventional, non-radiation hardened P channel FET also shifts more negative as it is exposed to radiation. However, the initial threshold voltage is negative. Therefore, the gate threshold voltage never goes through a region where the FET is uncontrollable, it only goes from a negative value to a more negative value. Therefore, conventional P channel FETs could be more robust to total dose effects than conventional N channel FETs if the proper gate drive signal is provided.